Coolants (heat transfer solutions) are used to remove heat from engines, particularly in combustion engines of the automotive industry. In order to provide optimal efficiency to the engine, excess heat should be removed as quickly as possible without damaging or decreasing the operation of all cooling system parts. Much progress has been made towards the protection of the cooling system materials especially in the area of protection (passivation) against corrosion at high temperatures. Although, from a corrosion standpoint temperatures close to boiling are indeed very critical, the low temperature domain is also of high importance during engine operation. At temperature below the freezing point, not the corrosion protection but the solubility and low temperature pumpability is of major importance.
Ideally the coolant remains transparent and free of insoluble materials. Haziness, precipitation, or in extremes, gel formation are considered detrimental for the performance of an engine coolant. Deposit formation, on the one hand, will result in abrasive conditions and will physical damage soft materials in the cooling system. Problems resulting from instability can be seen in water pump seals, engine head seals, hoses or any other parts where softer materials are in use. Gel formation, on the other hand will have a negative impact on the viscosity and results in a negative change of the heat transfer characteristics of the fluid and can be observed in the heat exchangers of the cooling system.
In both heavy duty and off highway applications, the use of nitrite alone and in the combination with molybdate is still widespread for the protection of an engine's ferrous wet sleeve liner from cavitation. The use of such coolant formulations containing nitrite is widely spread in the United States, since many fleet and truck owners as well as the Truck Maintenance Council (TMC) have published recommended practice guidelines. TMC publishes the Recommended Practices Manual. This comprehensive manual contains more than 250 Recommended Practices (RPs). A Recommended Practice is a specification or practice, the adoption of which is voluntary. It is used to assist fleets and equipment manufacturers in the purchase, design, maintenance and performance of their equipment. TMC issues two types of RPs: Recommended Maintenance Practices, and Recommended Engineering Practices. Recommended Maintenance Practices are voluntary practices that assist equipment users, vehicle/component manufacturers, and other industry suppliers in the maintenance of commercial vehicle equipment. They also include informational documents that cover technical aspects of maintenance, equipment and supporting technologies. Recommended Engineering Practices are voluntary practices that assist equipment users, vehicle/component manufacturers, and other industry suppliers in the design, specification, construction and performance of commercial vehicle equipment.
Until a clear and differentiating test method exists that can provide guarantee for the protection against cavitation, a coolant which contains nitrite remains recommended to guarantee sufficient protection of the liners. The applicable recommended practice guideline is RP329, Fleet Purchasing Specification for Nitrite-Containing Ethylene Glycol Base Coolant. This Recommended Practice specifies requirements for concentrated and prediluted forms of fully-formulated ethylene glycol base antifreeze/coolant which does not require a precharge of supplemental coolant additives.
If a coolant formulation contains nitrite in the ranges set forth as follows, it is acceptable for wet liner cavitation protection to Original Equipment Manufacturers. This is based on the historically positive observations and conclusions that nitrite has shown over decades of its capability of providing ferrous alloys protection from cavitation damage.                Concentrated antifreeze w/coolant must contain either: A. at least 2400 ppm nitrite (as NO2−); or B. a combined total of at least 1560 ppm nitrite (as NO2−) plus molybdate (as MoO42−); with a minimum of 600 ppm of either. Prediluted antifreeze/coolant must contain either: A. at least 1200 ppm nitrite (as NO2−) or B. a combined total of at least 780 ppm nitrite (as NO2−) plus molybdate (as MoO42−); with a minimum of 300 ppm of either.        
In today's engine applications, large amounts of lightweight, Group III metals such as aluminum and its alloys are used as an alternative to other metals such as cast iron, copper, solder, brass, steel, magnesium, and their alloys in the construction of different components. Modern heat exchangers and water pumps are the best known examples of such components. There is particular interest in aluminum due to its lower weight combined with acceptable strength and efficient heat transfer properties.
In order to protect the cooling system and engine parts against corrosion, an additive package containing corrosion inhibitors is generally added to the coolant base fluid. Various corrosion inhibitors have been added to water/alcohol based coolants and heat transfer fluids to reduce corrosion of metallic systems. Carboxylate corrosion inhibitor combinations are well known, e.g. non-silicate antifreeze formulations containing alkali metal salts of benzoic acid, dicarboxylic acids and nitrate. Alternately, corrosion inhibitors comprising the combination of an aliphatic monoacid or salt, a hydrocarbyl dibasic acid or salt and a hydrocarbyl triazole are also known. A corrosion inhibitor using an alkylbenzoic acid salt, an aliphatic monoacid or salt and a hydrocarbyl triazole has also been disclosed, as have phosphate and nitrite free antifreeze formulations containing aliphatic monobasic acids or salts, an alkali metal borate compound and a hydrocarbyl triazole. Antifreeze compositions containing an aliphatic monoacid or salt, a hydrocarbonyl triazol and imidazol have also been described.
The addition of phosphonocarboxylates to coolants as a corrosion inhibitor in combination with inorganic phosphates, is known, combinations of phosphonocarboxylates and strontium or magnesium or calcium compounds were explored but are limited to the use of phosphonate 2-phosphonobutane-1,2,4-tricarboxylic acid or a salt thereof.
Recently, fast depletion of certain corrosion inhibitors such as nitrites, and silicates, has been observed in the field, together with an important pH increase. Those changes to the coolant can have a negative effect on its stability and long term performance. Test work conducted has indicated that these coolant changes occur in different type of heat exchangers, with both brazed and unbrazed surfaces. The changes observed in the coolant can be explained by the known chemical reaction of nitrites with unpassivated metals, such as aluminum, in the presence of sodium hydroxide. This results in the formation of ammonia and aluminum oxides and with a pH increase as result.
Earlier studies have indicated that reduction of nitrites and pH increase in the coolant could be inhibited by pre-washing the brazed surfaces of metals such as aluminum with a phosphate solution, neutral to slight alkaline, prior to contact with the coolant.
Because nitrite containing coolants are still widely used, certainly in the U.S. market, a need is present for nitrite containing coolant formulations that also provide long lasting corrosion protection for the increased aluminum surfaces in modern cooling system designs.